Endodontic therapy is a dental procedure, colloquially know as a “root canal”, undertaken to repair and save a tooth by removing and replacing infected dental pulp.
FIG. 1a show a cross-section of an exemplary healthy tooth 10. The main body 12 of the tooth is composed of dentin, a matrix of mineralized connective tissue, that supports an overlay of hard, brittle tooth enamel 14. The main body 12 of the tooth is supported by the gums 16 that cover the jaw bone 18, and also contains dental pulp 20, a soft connective tissue. Nerves in the dental pulp 20 connect with the rest of the body via one or more canals 22 located in the roots of the tooth 24.
When dental pulp 20 becomes irritated, it swells up. Since the dental pulp 20 is encased in a rigid matrix of dentin, there is little or no room for expansion and the nerves in the dental pulp 20 are squeezed or pinched, causing a great deal of pain. A “root canal” is an endodontic procedure designed to alleviate this pain and save the tooth by removing the dental pulp 20 and replacing it with a bio-inert material such as gutta-percha.
FIG. 1b shows the first step in a typical root canal procedure. A portion of the tooth enamel 14 has been removed, along with a portion of the main body 12 of the tooth and the bulk of the dental pulp 20. This removal is typically effected using a tungsten carbide, or diamond, tipped dental bur 26, a.k.a. a dental drill bit.
FIG. 1c shows a further step in a typical root canal procedure. At this stage, the dental pulp 20 has been removed from the right hand canal 22, and the dental pulp 20 is in the process of being removed from the left hand canal 22 by means of an endodontic instrument 28.
FIG. 1d shows a completed root canal treatment. The dental pulp 20 has been completely removed and replaced by a bio-inert material 34.
FIG. 2a shows an exemplary, prior art, endodontic instrument 28 used to remove dental pulp 20 from the canals 22 in a root canal procedure. The endodontic instrument 28 has a shank 42, a quick change, cam drill holder 44, a cylindrical shaft 46 and a flexible cutting bit 48. The cam drill holder 44 may be one of the standard handles for connecting the endodontic instrument 28 to an endomotor.
Prior art endodontic instruments 28 are typically made from a Nickel-Titanium (NiTi) alloy. NiTi alloys are more flexible than more conventional stainless steels, but are subject to metal fatigue and may break after repeated use, or after a number of flexures in a single use. Prior art endodontic instruments 28 also typically have screw shaped flexible cutting bits 48 that may result in the bit becoming “screwed in” to the canals 22, creating a situation where the flexible cutting bit 48 may be torsionally overloaded. Prior art endodontic instruments 28 typically have a tapered, or conical, flexible cutting bit 48 that narrows down from the proximate end of the bit to the distal end.
FIG. 2b shows a close-up view of the distal end of a prior art endodontic instrument 28. FIG. 2b clearly showing the screw shaped cutting edge 50.
FIG. 2c shows a close-up view of a cross-section of the flexible cutting bit 48 of a prior art endodontic instrument 28 showing the cutting edges 50.
FIG. 2d shows a close up view of the flexible cutting bit 48 of a traditional rotary NiTi endodontic instrument 28. FIG. 2d clearly shows the tapered, or conical, flexible cutting bit 48 that narrows down from the proximate end of the bit to the distal end.
In use, the endodontic instrument 28 may be attached by the cam drill holder 44 to a rotary drill or rotary endomotor. The endomotor, which may be an electrically powered drill, applies a torque to the endodontic instrument 28 that is transmitted via the handle the shank 42 to the flexible cutting bit 48. The applied torque results in a slight twisting of the flexible cutting bit 48 as the cutting edges 50 engage the dental pulp 20 in the canals 22 and the surrounding dentin in the main body 12.
FIG. 3a shows a plot of c(z), the torsional rigidity of a NiTi flexible cutting bit 48, measured in dyne·cm2, as a function of distance along the axis 52 of the flexible cutting bit 48, measured in cm.
The plot 56 is calculated for a NiTi flexible cutting bit 48 having a diameter D0 at the distal end 54 of 0.25 mm. The distal end 54 is also where z, the distance along the axis 52, is taken to be zero. The length L of the flexible cutting bit is 16 mm and the cone shape of the flexible cutting bit 48 is 2%, i.e., the radius of cross-section increases by 0.02 mm along each mm from the tip of instrument.
From plot 56, it is evident that the torsional rigidity of the flexible cutting bit 48 is at its least at the distal end 54, where the radius of the flexible cutting bit 48 is smallest.
FIG. 3b shows τ(z), the torsion angle per unit length, of the same NiTi flexible cutting bit 48, plotted as a function of distance z, measured in cm, along the axis 52 of the flexible cutting bit 48. τ(z), the torsion angle per unit length, is shown for two different values of applied torque when the tip of the endodontic instrument 28 is held stationary.
Plot 58 shows the torsion angle per unit length τ(z) of the typical, conical endodontic instrument 28 when a torque of 100 dyne·cm is applied to the shank 42 while the distal end 54 is held stationary.
Plot 60 shows the torsion angle per unit length τ(z) of the typical, conical endodontic instrument 28 when a torque of 150 dyne·cm is applied to the shank 42 while the distal end 54 is held stationary. These plots are based on mathematical analysis shown in detail in, for instance, U.S. Provisional Patent Application No. 61/231,474 filed on Aug. 5, 2009 by E. Rzhanov et al. titled “High Safety Files for Root Canal Treatment”, the contents of which are hereby incorporated by reference.
From these plots, it is evident that the torsion angle per unit length τ(z) will most probably first exceed some upper critical value in the vicinity of the distal end 54 of the endodontic instrument 28. This is where the flexible cutting bit 48 is located. The flexible cutting bit 48, particularly the distal end of the flexible cutting bit 48, is, therefore, where any excessive torque applied to the endodontic instrument 28 will most likely begin to deform the endodontic instrument 28, and it is also the region where any breakage is most likely to occur.
When a flexible cutting bit 48 breaks deep in a canal 22, it is often impossible to retrieve the broken portion. The broken, distal portion of the flexible cutting bit 48, therefore, often has to be left in place where it broke in the canal 22. This is not a very satisfactory outcome for the patient as it sometimes means that the tooth then has to be removed, which is what the root canal procedure was intended to avoid.
An endodontic instrument 28 with a cutting bit that is flexible, robust and not inclined to break is, therefore, highly desired. It is further highly desired that the endodontic instrument 28 is manufactured so that, if breakage does occur, the distal portion of the broken instrument may be easily removed from the patient's tooth.